1. Field of the Invention
The present invention relates to a method for producing a pigment dispersing resin varnish for cationic electrodeposition paint, a pigment dispersing resin varnish obtained therefrom, and a cationic electrodeposition coating composition using the pigment dispersing resin varnish.
2. Description of the Related Art
Recent increased awareness of the environment has led to controls on the amounts of harmful air pollutants (HAPs) in the more advanced nations. Cationic electrodeposition paint are aqueous paint based on water media, but a potential HAP component (for example, cellosolves, such as butyl cellosolve or ethyl cellosolve) is the solvent that is used during the production of the cationic epoxy resin upon the reaction of an amine, phosphine, or sulfide with the epoxy resin starting material, that is, during the onium conversion of the pigment dispersing resin, at the stage where the pigment dispersing resin is produced. Since the cellosolve solvents are readily volatilized, when such substances are contained in cationic electrodeposition paint, they run the risk of evaporating into the atmosphere and becoming a source of harmful air pollutants. The use of alternatives results in the risk of electrodeposition coating films with lower flow properties and a less attractive appearance.
An object of the present invention is to provide a method for producing a pigment dispersing resin varnish for cationic electrodeposition paint, wherein the flow properties and appearance of the resulting electrodeposition coating film are not compromised, despite the fact that no cellosolve solvent is used, as well as a cationic electrodeposition resin composition allowing volatile organic carbon compounds to be reduced (VOC reduction).
The method for producing a pigment dispersing resin varnish for cationic electrodeposition paint in the present invention comprises the step of using a solvent comprising a polyalkylene oxide compound represented by the following formula for producing a resin varnish which is obtained from a cationic epoxy resin composition having amino groups, phosphonium groups, or sulfonium groups. 
(where R is an ethylene group or propylene group; Ph is a phenylene group; and both n and m are a number of 1 or more).
The aforementioned solvent is preferably added to the cationic epoxy resin composition which has been obtained upon the reaction of an amine, phosphine, or sulfide with an epoxy resin.
Another way of using the solvent is to employ it as a reaction solvent during the production of the cationic epoxy resin composition upon the reaction of an amine, phosphine, or sulfide with an epoxy resin.
The epoxy resin is preferably a urethane-modified epoxy resin.
R in the formula for the polyalkylene oxide compound is preferably an ethylene group, and the total of n and m is preferably 2 or more but less than 20.
The content of the polyalkylene oxide in the solvent is preferably 5 to 100 wt %.
The pigment dispersing resin varnish of the present invention is obtained by the aforementioned method for the production of pigment dispersing resin varnishes for cationic electrodeposition paint, where the content of the polyalkylene oxide compound in the resin varnish is preferably 1 to 50 wt %. The cationic electrodeposition coating composition of the present invention comprises a pigment dispersing resin varnish obtained in this manner, and has a polyalkylene oxide compound content of 0.1 to 2.0 wt %.
The method for producing a pigment dispersing resin varnish for cationic electrodeposition paint in the present invention comprises the step of using a solvent comprising a polyalkylene oxide compound represented by the following formula for producing a resin varnish comprising a cationic epoxy resin composition having amino groups, phosphonium groups, or sulfonium groups: 
(where R is an ethylene group or propylene group; Ph is a phenylene group; and both n and m are a number of 1 or more). That is, in the method for producing the pigment dispersing resin varnish, when the amine, phosphine, or sulfide reacts with the epoxy resin described below to bring about onium conversion, that is, when amino, phosphonium, or sulfonium groups are introduced, the epoxy resin is first dissolved in a solvent comprising the aforementioned polyalkylene oxide compound, or the aforementioned solvent is added to the cationic epoxy resin composition obtained following the aforementioned onium conversion reaction, so as to produce the pigment dispersing resin varnish. The amine, phosphine, or sulfide that is added reacts with the epoxy groups present in the epoxy resin, to introduce the onium groups into the epoxy resin.
Examples of the aforementioned epoxy resin generally include polyepoxides. The epoxides have an average of two or more 1,2-epoxy groups per molecule. The polyepoxides should have 180 to 1,000 epoxy equivalents, and preferably 375 to 800 epoxy equivalents. Fewer than 180 epoxy equivalents will not allow a film to be formed during electrodeposition, and thus will not allow a coating film to be obtained. More than 1,000 will result in an insufficient amount of onium groups per molecule, and thus in insufficient water solubility.
Useful examples of the polyepoxides include polyglycidyl ethers of polyphenols (such as bisphenol A). These can be prepared by etherifying a polyphenol with epichlorohydrin or dichlorohydrin in the presence of an alkali. The polyphenols can be bis(4-hydroxyphenyl)-2,2-propane, 4,4xe2x80x2-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, or similar materials.
The aforementioned epoxy resins may be epoxy resins containing oxazolidone rings in the resin skeleton, in the epoxy resin in the main emulsion described below.
Epoxy resins containing hydroxyl groups in particular may be urethane-modified epoxy resins with blocked isocyanate groups introduced by reaction of half-blocked isocyanates with the hydroxyl groups.
The half-blocked isocyanates used for reactions with the aforementioned epoxy resins may be prepared by partial blocking of organic polyisocyanates. The reaction between the organic polyisocyanates and blocking agents is preferably carried out while the material is cooled to between 40 and 50xc2x0 C. as the blocking agent is added in the form of drops while stirred in the presence of a tin catalyst as needed.
The aforementioned organic polyisocyanates can be any having two or more isocyanate groups per molecule. Specific examples include aliphatic compounds such as trimethylene diisocyanate or hexamethylene diisocyanate; alicyclic compounds such as 1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate, or isophorone diisocyanate; aromatic compounds such as 2,4-tolylene diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate, or 1,4-naphthalene diisocyanate; and polyisocyanates such as dimers or trimers thereof.
Lower aliphatic alkyl monoalcohols with 4 to 20 carbon atoms are suitable blocking agents for preparing the aforementioned half-blocked isocyanates. Specific examples include butyl alcohol, amyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, heptyl alcohol, and eicosanol.
The reaction between the aforementioned epoxy resin and half-blocked isocyanates is carried out by being held for about 1 hour preferably at 140xc2x0 C.
R in the formula for the aforementioned polyalkylene oxide compounds is preferably an ethylene group, and the total of n and m is preferably 2 or more but less than 20, and even more preferably 2 to 10. A total of less than 2 results in greater susceptibility for volatilization into the atmosphere, while more than 20 runs the risk of resulting in an electrodeposition coating film that is unattractive.
The aforementioned solvents preferably contain 5 to 100 wt % polyalkylene oxide compound. That is, at a content of 5 wt % or more, the solvent may itself be the aforementioned polyalkylene oxide compound. A polyalkylene oxide compound content of less than 5 wt % complicates the effort to achieve VOC reduction. Examples of solvents other than the aforementioned polyalkylene oxide compounds which the aforementioned solvent may contain can include solvents commonly used in the synthesis of resins for electrodeposition paint, for example, ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, 3-methyl-3-methoxybutanol, diethylene glycol monobutyl ether, and dipropylene glycol monobutyl ether; and alcohols such as butanol.
The content of the polyalkylene oxide compound in the aforementioned solvents is 5 to 100 wt %, as noted above, but the preferred range of the aforementioned polyalkylene oxide compound content is based on differences in the types of onium groups introduced to the epoxy reins, that is, primary amino groups, quaternary ammonium groups, sulfonium groups, or phosphonium groups. For example, the range is 5 to 50 wt % for primary amino groups, 5 to 100 wt % for quaternary ammonium groups, and 5 to 100 wt % for sulfonium groups.
The onium conversion reaction of the epoxy resins is carried out by dissolving the epoxy resin in the aforementioned solvent to allow the amine, phosphine, or sulfide to react with the epoxy groups. The details are described in further detail below
Primary amino groups or quaternary ammonium groups can be introduced to allow amines to react with an epoxy resin.
In the case of the former, when the reaction is brought about by directly adding the primary amine to the epoxy resin solution, the primary amino groups themselves end up reacting with the epoxy groups, so a polyamine with primary amino groups and secondary amino groups is used to allow a compound obtained by the ketimination of the primary amino groups to react with the epoxy groups in an epoxy resin dissolved in the aforementioned solvent. After the reaction, the ketimine blocks are removed, the primary amino groups are reproduced, and an ammonium salt of a primary amine is produced by neutralization.
Examples of polyamine compounds at such times include diethylenetriamine, aminoethyl ethanolamine, and aminoethyl piperazine. These polyamines are ketiminated by reaction with a ketone such as acetone, methyl ethyl ketone, or methyl isobutyl ketone. The ketimination reaction will readily progress when heated to 100xc2x0 C. or higher to distill off the water that is produced.
The reaction between the partial ketimine compound and epoxy resin is held for 1 hour at 120xc2x0 C. and then cooled to 90xc2x0 C., a suitable amount of pure water is introduced, and the ketiminated primary amino groups are reproduced. The amount of the polyamine and epoxy resin used here is preferably an equivalent ratio of 1/2 to 1.2/1.
When quaternary ammonium groups are introduced, on the other hand, a neutral acid salt of a tertiary amine is allowed to react with the epoxy groups of the epoxy resin. The tertiary amine should have 3 to 6 carbon atoms, and may have hydroxyl groups. Specific examples of tertiary amines include dimethylethanolamine, trimethylamine, triethylamine, dimethylbenzylamine, diethylbenzylamine, N,N-dimethylcyclohexylamine, tri-n-butylamine, diphenethylmethylamine, dimethylaniline, and methylmorpholine.
The reaction between the epoxy resin and the neutral acid salt of a tertiary amine can be carried out in the usual manner. For example, a solution comprising the epoxy resin dissolved in the aforementioned solvent is heated to between 60 and 100xc2x0 C., a tertiary amine is added thereto, and the reaction is carried out for 2 to 10 hours. Examples of neutral acids include organic acids or inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid.
When the onium conversion is managed through the reaction of a sulfide, sulfonium groups are introduced by bringing about a reaction between a sulfide and the epoxy groups in the epoxy resin. Specifically, the reaction is carried out at a temperature of 70 to 750xc2x0 C. by mixing and stirring water, a sulfide neutral acid, and epoxy resin dissolved in the aforementioned solvent. Examples of sulfides include aliphatic sulfides, mixtures of aliphatic and aromatic sulfides, aralkyl sulfides, and cyclic sulfides. Specific examples include 1-(2-hydroxyethylthio)-2-propanol, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, diphenyl sulfide, dihexyl sulfide, ethylphenyl sulfide, tetramethylene sulfide, pentamethylene sulfide, thiodiethanol, thiodipropanol, and thiodibutanol. The amount of sulfide and epoxy resin is preferably an equivalent ratio of 1/1 to 5/1.
Finally, when onium conversion is managed through the reaction of a phosphine, the phospine reacts with the epoxy groups in the epoxy resin. Specifically, a phosphine, and preferably an organic phosphine, is allowed to react with an epoxy resin dissolved in the aforementioned solvent. The reaction conditions are the same as those for sulfides.
The pigment dispersing resin varnish for cationic electrodeposition paint in the present invention is obtained by the method described above. The resin varnish contains 1 to 50 wt % polyalkylene oxide compound. Less than 1 wt % makes VOC reduction more difficult, whereas more than 50 wt % runs the risk of resulting in lower corrosion resistance. A pigment and the resulting pigment dispersing resin varnish are dispersed in an aqueous medium, giving a pigment dispersion paste.
The aforementioned pigment can be any commonly used pigment. Examples include coloring pigments such as titanium white, carbon black, and red oxide; body pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica, clay, and silica; and rust-proof pigments such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, and aluminum phosphomolybdate.
The pigment dispersing resin varnish and the pigment, in an amount of 10 to 1000 weight parts per 100 weight parts resin solids, are mixed and dispersed using a common dispersing device such as a ball mill or sand grind mill until the particle diameter of the pigment in the mixture is the desired mean particle diameter, giving a pigment dispersion paste.
The resulting pigment dispersion paste and a separately prepared amine-modified epoxy resin, blocked polyisocyanate curing agent, and neutralizing agent can be dispersed in an aqueous medium to obtain a cationic electrodeposition paint.
The amino-modified epoxy resin is described in detail below.
Amino-modified epoxy resins are well-known resins used in common cationic electrodeposition paint, the details of which have been disclosed in Japanese Patent Publication (Kokoku) Nos.S55-34238, S56-34186, and S59-15929. Generally used amine-modified epoxy resin have a molecular weight of 600 to 8,000, an amine value of 16 to 230, and epoxy equivalents ranging from 300 to 4,000.
Typically, all of the epoxy rings of bisphenol type epoxy resin undergo ring-opening with an active hydrogen compound allowing the introduction of cationic groups, or some of the epoxy rings undergo ring-opening with other active hydrogens, and the remaining epoxy rings undergo ring-opening with active hydrogen compounds allowing the introduction of cationic groups.
Typical examples of bisphenol types of epoxy resins include bisphenol A or bisphenol F types of epoxy resins. Examples of commercially available products of the former include Epikote 828 (by Petrochemical Shell Epoxy, 180 to 190 epoxy equivalents), Epikote 1001 (same, 450 to 500 equivalents), and Epikote 1010 (3000 to 4000 epoxy equivalents). Examples of commercially available products of the latter include Epikote 807 (same, 170 epoxy equivalents).
Primary amines and secondary amines are active hydrogen-containing compounds allowing the introduction of cationic groups. Examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, as well as secondary amines comprising blocked primary amines such as ketimines of aminoethylethanolamine, and diketimines of diethylenetriamine. Such amines may be used in combination.
Examples of other active hydrogen-containing compounds which can be used for ring-opening of the aforementioned epoxy rings include monophenols such as phenol, cresol, nonylphenol, and nitrophenol; monoalcohols such as monobutyl or monohexyl ethers of propylene glycol or ethylene glycol, stearyl alcohol, 2-ethylhexanol, or hexyl alcohol; aliphatic monocarboxylic acids such as stearic acid and octylic acid; aliphatic hydroxycarboxylic acids such as glycolic acid, dimethylolpropionic acid, hydroxypivalic acid, lactic acid, and citric acid; and mercapto alkanols such as mercaptoethanol.
Epoxy resins containing oxazolidone rings in the resin skeleton, such as those disclosed in Japanese Unexamined Patent Applications (Kokai) Nos. H5-306327, H6-329755, and H7-33848, are preferred as the aforementioned amine-modified epoxy resins. Amine-modified epoxy resins containing such oxazolidone rings are described in further detail.
It is well known that epoxy resins with extended chains containing oxazolidone rings can be obtained when bifunctional epoxy resins are allowed to react with monoalcohol-blocked diisocyanate compounds, that is, bisurethane. Amine-modified epoxy resins obtained by ring-opening the epoxy rings of epoxy resins with amines are an example of amine-modified epoxy resins containing such oxazolidone rings. Modified epoxy resins containing oxazolidone rings can be obtained when a bifunctional epoxy resin is allowed to react with an asymmetrical bisurethane compound that is obtained when one of the isocyanate groups of a diisocyanate compound has been reversibly blocked with a monoalcohol, and the other isocyanate group has been irreversibly blocked with a hydroxyl group-containing compound, in accordance with the method disclosed in Japanese Unexamined Patent Application (Kokai) No. H7-33848. A cationic modified epoxy resin is obtained when the epoxy rings of the resulting modified epoxy resin are opened with an active hydrogen compound permitting the introduction of cationic groups, such as an amine.
Hydroxyl compounds to be employed when the other isocyanate group of a diisocyanate compound is irreversibly blocked in this method are C4 or higher aliphatic monoalcohols such as butanol and 2-ethylhexanol, long-chain alkyl phenols such as nonylphenol, and glycol monoethers such as mono-2-ethylhexyl ether of ethylene glycol or propylene glycol.
Examples of blocked polyisocyanate curing agents include those commonly used in the field, for example, aromatic diisocyanate compounds such as tolylene diisocyanate (TDI), 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), and xylylene diisocyanate (XDI); aliphatic or alicyclic diisocyanate compounds such as hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), 2,5- or 2,6-bis(isocyanatemethyl)bicyclo[2,2,1]heptane (norbornane diisocyanate NBDI); and blocked polyisocyanate compounds such as trimethylolpropane adducts and dimers or trimers of such diisocyanates.
The blocking agents used for the blocked polyisocyanate curing agent include those which undergo addition to isocyanate groups and are capable of reproducing isocyanate groups that are stable at ordinary temperature but are free when heated to or beyond the dissociation temperature.
Specific examples include phenol blocking agents such as phenol, cresol, xylenol, chlorophenol, and ethylphenol; lactam blocking agents such as xcex5-caprolactam, xcex4-valerolactam, xcex3-butyrolactam, and xcex2-propiolactam; active methylene blocking agents such as ethyl acetoacetate and acetylacetone; alcohol blocking agents such as methanol, ethanol, propanol, butanol, amyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, benzyl alcohol, methyl glycolate, butyl glycolate, diacetone alcohol, methyl lactate, and ethyl lactate; oxime blocking agents such as formaldoxime, acetoaldoxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, and cyclohexane oxime; mercaptan blocking agents such as butyl mercaptan, hexyl mercaptan, t-butyl mercaptan, thiophenol, methylthiophenol, and ethylthiophenol; acid amide blocking agents such as succinic acid imide and benzamide; imide blocking agent such as imide succinate and maleic acid imide; and imidazole blocking agents such as imidazole and 2-ethyl imidazole. The use of lactam and oxime blocking agents is preferred when low temperature curing of 160xc2x0 C. or lower is desired.
The aforementioned neutralizing agents are not particularly limited and are the same as those used to produce the aforementioned resin varnish for pigment dispersion. Specific examples include organic or inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid.
The cationic electrodeposition coating composition of the present invention is produced in the following manner. That is, the aforementioned amine-modified epoxy resin and a blocked polyisocyanate curing agent are mixed to homogeneity in the prescribed amounts, and the resulting mixture is then dispersed in an aqueous medium containing a neutralizing agent, giving a mixture emulsion (hereinafter referred to as xe2x80x9cmain emulsionxe2x80x9d) of the amine-modified epoxy resin and the blocked isocyanate curing agent. The main emulsion, the aforementioned pigment dispersion paste, and deionized water are then mixed in the prescribed amounts, giving the cationic electrodeposition paint of the present invention.
The amount of the blocked polyisocyanate curing agent should be sufficient to react with the active hydrogen-containing functional groups such as amino groups or hydroxyl groups in the amine-modified epoxy resin when heated and cured, so as to provide a suitably cured film. The weight ratio of the amine-modified epoxy resin to the blocked polyisocyanate curing agent ranges from 90/10 to 50/50, and preferably from 80/20 to 65/35.
The aforementioned pigment dispersion paste is blended in such a way that the pigment is 1 to 35% relative to the total resin solids weight of the cationic electrodeposition paint. The cationic electrodeposition paint thus produced should contain 0.1 to 2 wt % polyalkylene oxide compound. Less than 0.1 wt % makes it difficult to reduce the VOCs, whereas more than 2 wt % runs the risk of lower corrosion resistance.
The cationic electrodeposition paint of the present invention can contain a tin compound such as dibutyltin dilaurate or dibutyltin oxide, or a common urethane cleavage catalyst. The amount that is added should be 0.1 to 5.0 wt % of the blocked polyisocyanate curing agent.
The cationic electrodeposition paint of the present invention can also include common paint additives, such as water-miscible organic solvents, surfactants, antioxidants, and UV absorbents.